Mode selector for scan laser

ABSTRACT

The invention relates to a mode selector having a plate showing an electro-optical effect in an electric field to control the modes of oscillation in a scan laser. The new mode selector consists of two identical selectors which are rotated relative to each other through 90* and which are each arranged in a focal plane of the laser-resonator and of which one controls the x coordinate and the other controls the y coordinate of the focus, in which a plate-shaped electro-optical KDP material is surrounded by grid-shaped electrodes between which electric fields are produced which, with the exception of a strip-shaped region, everywhere exceed a certain minimum intensity but disappear or are very small in the strip-shaped region. The electrodes are two grids of parallel wires and the individual wires have such a potential that either a transverse or a longitudinal electro-optical effect occurs in the electro-optical material.

United States Patent [191 Schulten Aug. 14, 1973 i 1 MODE SELECTOR FORSCAN LASER Primary Examiner-william L. Sikes [75 Inventor: GiinterSchulten, Wedel, Germany A"mey Frank Tnfa [73] Assignee: U.S. PhilipsCorporation, New [57] ABSTRACT N The invention relates to a modeselector having a plate [22] Filed: 'June 2, 1972 showing anelectro-optical effect in an electric field to control the modes ofoscillation in a scan laser. The [21} App! 259191 new mode selectorconsists of two identical selectors which are rotated relative to eachother through 90 [30] Foreign Application Priority Data and which areeach arranged in a focal plane of the las- June 3, 1971 Germany P 21 27463.5 and which the coordinate and the other controls the y coordinateof 52] us. c1. 350/160, 331/945 the in which a Plate-shaped eiwmhpiicai511 110. C1. G02! 1/26 maieriai is suimuhdeii by grid-shaped electrodes[58] Field of Search 331/943; 350/160, twee" which eieciric fieldsPmduced which, with 350/147 150 the exception of a strip-shaped region,everywhere exceed a certain minimum intensity but disappear or are [56']References Cited very small in the strip-shaped region. The electrodesUNITED STATES PATENTS are two grids of parallel wires and the individualwires D have such a potential that either a transverse or a longig gz etah 62 8 tudinal electro-optical effect occurs in the electroopticalmaterial.

4 Claims, 4 Drawing Figures PATENTEDMIB 14 ms" sum 2 or 3 Fig.2

m m i s O "U -U Fig.3

I 1 MODE SELECTOR FOR SCANLASER The invention relates to a mode selectorhaving a plate which shows an electro'opticaleffect in an electric fieldto control the modes of oscillation ina scan laser.

A scan laser is a laserwhose output beam can selectivelybe switchedinvarious directions or be moved parallel in various positions. Like aconventional laser, such a laser consists of an active medium whichproduces an amplification of the light and a resonator which ensures thefeedback coupling andthus produces a self-excitation. The. resonatormust have a particular property. It must be degenerated; this means thatit must be capable of oscillating in various modes.

of the same frequency. FIG. l-showssuch a resonator. Two lenses 1 and 1'having afocal length f are at a distance 2f apart. The. active medium 2is present between said lenses. Oneither side, parallel flat mirrors 3,3' are arranged at a distance f from the lenses. Several modes arepossible which differ by the angle a between optical axis and centralaxis m of the mode. In FIG. 1 two such modes are shown by their axes andtheir lateral boundary lines (see Meyers, R.A., Pole, R.V.: The electronbeam Scanlaser: Theoretical. and Operational Studies, IBM Journal Sept.,1967, 502 A moreexact examination into the mode theory provides thefollowing relationship: in the central plane 4'between the two lenses 1,I, all modes have a waist ofdiameter 2 w, and in the focal plane 3 onthemirrors likewise a waist of diameter 2 01 (FIG. lb). Between w, andw, the relationship exists m to, f M11, wherein f is the focal length ofthe lenses and A the wavelength. For a high resolution of the scan laser(a maximum of foci in the focal plane separated spatially by d and h,respectively), (0 should be as smallas possible. An optimum value forin, results from the dimensions of the active medium.

The resonator shown in FIG. 1 is only one form of a scanlaser-resonator. Resonators have also been proposed in which the focalplanes lie outside the mirror planes. This has the advantage thatunavoidable small errors on the mirrors have no influence on the qualityof the resonance.

The device shown in FIG. 1 becomes a scan laser in that various modeswhich differ by the angle a can be arbitrarily excited, or in otherwords: it should be possible to excite modes whose foci lie indifferentselected places of the focal plane with a distance from each other. Inthe known scan laser, a mode selector is arranged for that purpose inone of the focal planes (that is to say in the plane of one of themirrors of FIG. I). This is a device which controls the reflectionfactor of the mirror so that each time only a certain range having adiameter of approximately 2 m, is highly reflecting, the remainingmirror is poorly reflecting (for example, less than 80 percent). Thisoccurs as follows: before the mirror is arranged a flat plate of anelectrooptical material (KDP) and in addition a double refracting flatplate (quartz). The quartz plate produces a rotation of the plane ofpolarisation of the light. However, due to the Brewster windowsterminating the discharge vessel (it comprises the active plasma) thelaser can oscillate only in a certain direction of polarization. Arotation of the plane of polarization therefore results in losses oflight on the way there and back. Therefore the laser will not oscillate.By means of an electron beam, an

electric charge is now formedin the desired place of the electro-opticalmaterial. It produced an electric field in the material. Asa result ofthis, a rotationof the plane of polarization via the so-calledPockels-effect is used which is opposite to that resul'tin'g fro mquartz, In the case of a correctly chosenifieldstrength, th s "Q to say.in the case of the correctcharge density, overall rotation willdisappeararid the lossesare mini mum. The quartz then oscillates inamode which has a focus in the selected place. h

The device consisting of the KDP -,plate, the quartz plate and thecathode-ray tube thusrepresents a modu' lator whichcontrols thetransmission of light locally and in time. This device is called opticselector or mode selector. r

It is the object of the invention to improve a mode selector.Theinvention is based on the idea that every scanlaser-resonator has atleast two focal planes. It is therefore not necessary to control only inone focal plane the place of the focus indicated for example by thecoordinates x and y. Instead, the invention consists in that the modeselector consists of two identical selectors rotated through relative toeach other and each arranged in a focal plane of the laser-resonatorandof, which one controls the x coordinateand the other controls they-coordinate of the focus. According. to a further embodiment of theinvention the electric field strengths for the control are not producedby means of an electron beam but by a system having metallic electrodes.This is particularly simple since the. system is only two-dimensional sothat the electrodes may be cylindrical.

In an embodiment of the invention, the individual sclector shown in FIG.2 is constructed as follows:

A plane parallel layer or plate 5 of electro-optical material (KDP,Nitrobenzene, liquid crystals or other materials which show anelectro-optical effect), is provided on either side with two equal grids6, 6' consisting of parallel wires. The wires should be as thin aspossible and have a spacing distance of approximately 2 0: The two gridsare arranged so that each time two wires are located accurately oppositeto each other. The electro-optical plate 5 is oriented so that norotation of the polarization of the light which traverses the plate atright angles occurs when the electric field strength is failing. Inorder to improve the transmission, both sides of the plate may beprovided with layers reducing the reflection.

In accordance with the electro-optical material used, longitudinal (i.e.in the direction of the light path) or transverse electric fieldcomponents may be produced in accordance with the fact whether the wires6,6 of a grid have the same or alternately opposite potentials. In thecase of longitudinal fields, the control occurs in the manner shown inFIG. 3. The wires of one grid 6 have a potential +U, those of the othergrid 6 have a potential U. Only two selected adjacent wires and the twooppositely located wires have the potential zero. The whole plate 5 isthen traversed by electric fields and the electro-optical effectproduces a rotation of polarization and thus losses. An exception formsonly a strip-shaped region 5' in which no fields occur and which isbounded by the four electrodes of potential zero. No rotation occursthere and. thus no attenuation of a lightwave polarized by the Brewsterwindows.

By switching the potentials on other wires, the stripshaped region canbe moved, selection being possible.

FIG. 4 shows another embodiment of a mode selector. The electro-opticallayer 5 is surrounded by four cylindrical electrodes 7, 7', 8, 8'. Avoltage +U and U,, respectively, is set up at the two electrodes 7, 8,and accordingly a voltage U and +U respectively, is set up at the twoelectrodes 7', 8'. The sign of the voltages is chosen to be so that aquadrupole field occurs. Such a field is strongly inhomogeneous and hasin its centre 5' a field-free region, when U,=U If on the contrary U Uthere will still be a field-free region but it will be shifted laterallyin the direction of the electrodes having the smaller voltage. In thismanner, a strip-shaped region which is free from electric fields insidethe electro-optical material can be locally moved along the line 9 byvariation of the electrode voltage. Since only within said region therotation of the plane of polarization does not occur, the coordinate ofthe focus in one of the two focal planes is thus fixed. The width of theregion in which no rotation of polarization occurs, is accordinglysmaller as the values of the voltages U and U are larger.

What is claimed is:

1. In a mode selector device, for use in a scan laser for controllingmodes of oscillation in said laser, the subcombination comprising aplate-like material having an electro-optical effect under the influenceof an electric field, grid-shaped electrodes substantially surroundingsaid plate-like material for providing electric fields for saidmaterial, and means for supplying a potential to said grids to form afield exceeding a certain minimum intensity with exception ofastrip-shaped region of said material wherein said electric fields arediminished and are capable of being nonexistent in said region.

2. The mode selector device of claim 1, wherein said electrodes comprisetwo grids of substantially parallel wires, and further comprising meansfor impressing each of said wires with an individual potential whereinthe two grids can be provided with a similar field with respect to eachother.

3. The mode selector device of claim 1, wherein said electrodes comprisetwo grids of substantially parallel wires, and further comprising meansfor impressing each of said wires with an individual potential whereinthe two grids can be provided with an opposite field with respect toeach other.

4. In a mode selector device for use in a scan laser for controllingmodes of oscillation in said laser, the subcombination comprising aplate-like material having an electro-optical effect under the influenceof an electric field, four cylindrical electrodes arranged about saidmaterial so as to form an electric quadrupole field for said plate-likematerial, and means for impressing said electrodes with a varyingpotential, so as to create a movable, centrally located, field-freeregion in said material.

1. In a mode selector device, for use in a scan laser for controllingmodes of oscillation in said laser, the subcombination comprising aplate-like material having an electro-optical effect under the influenceof an electric field, grid-shaped electrodes substantially surroundingsaid plate-like material for providing electric fields for saidmaterial, and means for supplying a potential to said grids to form afield exceeding a certain minimum intensity with exception of astripshaped region of said material wherein said electric fields arediminished and are capable of being non-existent in said region.
 2. Themode selector device of claim 1, wherein said electrodes comprise twogrids of substantially parallel wires, and further comprising means forimpressing each of said wires with an individual potential wherein thetwo grids can be provided with a similar field with respect to eachother.
 3. The mode selector device of claim 1, wherein said electrodescomprise two grids of substantially parallel wires, and furthercomprising means for impressing each of said wires with an individualpotential wherein the two grids can be provided with an opposite fieldwith respect to each other.
 4. In a mode selector device for use in ascan laser for controlling modes of oscillation in said laser, thesubcombination comprising a plate-like material having anelectro-optical effect under the influence of an electric field, fourcylindrical electrodes arranged about said material so as to form anelectric quadrupole field for said plate-like material, and means forimpressing said electrodes with a varying potential, so as to create amovable, centrally located, field-free region in said material.